I'm trying to create shooting target-style single-color images from digital pictures of animals. Once I've lifted the image from the background, what are ways I could turn it into a simple line drawing?
I would be satisfied with something that got me edges even if I had clean up within them.
I'm assuming you are using Photoshop. You didn't tag your question with that, or mention it, but since it's the most popular tool...
If the background is transparent:
Command/Ctrl-click the layer thumbnail in the Layers Panel to create a selection.
Click the "Make Work Path" button on the bottom of the Paths Panel.
From the menu choose Layer > New Fill Layer > Solid Color
Pick the color you want the shape to be and hit OK.
You have a new Shape layer of the outline. You can then turn off or discard the previous layer. Then use the Path tools to clean up as needed.
If you want this in a vector application such as Illustrator:
Double-click the "Work Path" in the Paths Panel and give the path a name.
Choose File > Export > Paths to Illustrator..., select the path name, and hit OK.